Flounder

I had to pick up a couple flounder at my local oddball fishstore, I love em.
The guy told me that he had no idea what species they were because there are
soo many species. He also had no idea of how large they'd grow, though he
said that he never saw one any bigger than 5".

So, how do I identify these suckers (pun intended)? Or, where is there a
good website where I might read up on these boogers?

-Muuurgh